U. Benedict

Comparative aspects of the high-pressure behaviour of lanthanide and actinide compounds

Abstract All lanthanide and actinide metals which are available as solids in sufficient quantity have undergone structural investigation at elevated pressure and can be systematically compared. Systematics is much less complete for the pressure behaviour of the compounds. We shall see that sufficient information is only available for some of the 1:1 compounds with elements from the fifth and sixth main groups of the periodic table, the monopnictides and the monochalcogenides. This is in part due to radioactivity and scarcity of some of the elements concerned, but also to lack of pressure studies on compounds of some of the more easily accessible lanthanides. The available information on high-pressure structures, transition pressures and compressibilities is presented in the form of graphs and tables. The paper centres on a comparison of the CeX compounds with ThX, on one hand, and with PuX, on the other, where the largest amount of data has been obtained.

Structural data of the actinide elements and of their binary compounds with non-metallic elements

Abstract This paper covers crystal structure data of the actinide elements and of binary compounds of actinides (An) with non-metallic elements (X). A few ternary compounds closely related to binary ones are included. The compounds are classified according to X:An ratios and to structure types. Tables list the known compounds and their lattice parameters for each structure type.

Bulk moduli and high-pressure phases of the uranium rocksalt structure compounds-I. The monochalcogenides

Abstract The high-pressure crystal structures of the compounds UX, where X = S, Se and Te, have been studied using X-ray diffraction in the pressure range up to about 60 GPa. A rhombohedral distortion is observed for US above 10 GPa. Use and UTe transform to the CsCl structure at about 20 GPa and 9 GPa, respectively. The latter transformations show a considerable hysteresis when releasing the pressure. The scaling behaviour of the bulk modulus has been studied. It is shown that a log-log plot of the bulk modulus versus specific volume for the cubic phases gives a straight line with a slope near - 2.

Delocalisation of 5f electrons in berkelium and californium metals under pressure

Berkelium and californium were studied by X-ray diffraction up to pressures of 57 GPa (Bk) and 48 GPa (Cf). Participation of the 5f electrons in the metallic bonding, as evidenced by the appearance of the low-symmetry alpha -uranium structure and by a sudden volume decrease, starts at about 25 GPa in berkelium and at 41 GPa in californium.

A new high-pressure phase of uranium nitride studied by X-ray diffraction and synchrotron radiation

High-pressure X-ray diffraction studies have been performed on UN powder for pressures up to 34 GPa using synchrotron radiation and a diamond anvil cell. For the cubic low-pressure phase the bulk modulus B0 equals 203(6) GPa and its pressure derivative B′0 equals 6.3(6) in good agreement with other data from the literature. The UN material has been found to transform to a new phase, UN III, at 29 GPa. The transformation is first order with a 3.2% decrease in volume. The UN III phase has been indexed according to a face-centred rhombohedral cell with a = 4.657(5) A and α = 85.8(2)° at 34 GPa. The influence of the 5f electrons in the transformation is discussed.

Actinide compounds under pressure

Abstract An overview of pressure-induced structural phase transitions and compressibility of actinide compounds will be given. Systematic trends in the nature of the high-pressure phases, the transition pressures, the hysteresis to retransformation on pressure release, and the compressibility are observed in the family of AnX compounds of B1 (NaCl) structure type. The dioxides studied up to now form high-pressure phases of PbCl2 type. UX2 compounds of Fe2As type also tend to have PbCl2 type high-pressure phases. The Th3P4 type compounds studied up to now did not transform up to 50 GPa. The same is true for ThOS and UOSe up to about 45 GPa. Comparison with rare earth compounds will be made where possible.

X-ray diffraction study of protactinium metal to 53 GPa

Abstract High purity protactinium was studied by X-ray diffraction in a diamond anvil cell up to 53 GPa. The body-centred tetragonal structure was retained through the whole pressure range. A bulk modulus B 0 = 157(5) GPa and a pressure derivative B ′ 0 = 1.5(0.5) were deduced from a Birch fit of the combined data of two independent series of measurements. While the bulk moduli determined by the same method for the neighbouring elements Th and U are in relatively good agreement with those calculated by the LMTO method, the experimental bulk modulus for protactinium is higher than the calculated one by about 40%. The high bulk modulus suggests high 5f participation in the bonding of protactinium metal.

X-ray diffraction of berkelium metal under pressure to 57 GPa

Abstract An investigation of the structural behavior of berkelium metal under pressure has been carried out up to 57 GPa. Evidence for a sudden decrease in volume (“collapse”) of the berkelium metal with increasing pressure up to 22 GPa was not observed. Instead, three different metal phases were observed over the range of applied pressure. The initial double-hexagonal close-packed structure of the berkelium metal first transformed to an f.c.c. structure at 8 GPa, which above 22 GPa converted to a third phase. Diffraction data obtained from this latter phase can be indexed on the basis of the α-U-type (orthorhombic) structure. A bulk modulus of 30(10) GPa was estimated from the relative volume V V 0 and pressure data for berkelium metal below 22 GPa. (The error is given in parentheses.)

Delocalisation of 5f electrons in curium metal under high pressure

Curium metal was studied by X-ray diffraction up to 52 GPa. Starting with the double-hexagonal close-packed structure (Cm I), a cubic close-packed structure (Cm II) was formed at 23 GPa and another high-pressure phase (Cm III) at 43 GPa. The latter phase can be indexed as an orthorhombic cell of the alpha -uranium-type structure. The volume difference between the cell of Cm III and that of Cm II, together with the formation of a low-symmetry (orthorhombic) crystal structure, suggests that itineracy of the 5f electrons in curium metal starts at 43 GPa. The fact that the delocalisation pressure is higher in curium than in the neighbouring americium, berkelium and californium metals is explained qualitatively on the basis of a comparison of localisation energies.

Phase transformation of the monochalcogenides SmX (X S, Se, Te) under high pressure

The high-pressure behaviour of the monochalcogenides of samarium (SmS, SmSe and SmTe) was investigated up to pressures of 55 GPa using the energy dispersive X-ray diffraction technique and synchrotron radiation. SmTe and SmSe show abnormal volume changes around 5 and 7 GPa respectively. SmS shows a phase transformation at low pressure (less than 1.8 GPa), retaining the same cubic structure, but undergoing a considerable volume collapse and a change of colour. SmS and SmSe both exhibit a phase transition from NaCl- to CsCl-type structure at high pressure, which has not been observed previously.